Characteristics and Influencing Factors of Supercritical Methane Adsorption in Deep Gas Shale: A Case Study of Marine Wufeng and Longmaxi Formations from the Dongxi Area, Southeastern Sichuan Basin (China)

Deep shale reservoirs with burial depths of >3500 m are now the target of gas producing intervals in the Sichuan Basin, China. Due to the in situ high temperatures and pressures, methane in deep shale formations is supercritical, resulting in the inapplicability of some conventional adsorption models. To determine the methane adsorption capacity of deep shale and its influencing factors, different shale lithofacies from the Dongxi area of the southeastern Sichuan Basin are investigated and compared in this work. The nanopore structure of Wufeng (WF) and Longmaxi (LMX) gas shales are characterized using gas (nitrogen and carbon dioxide) physisorption and high-resolution scanning electron microscopy (SEM). Methane isothermal adsorption experiments are carried out on typical shale samples at different temperatures (from 30 to 90 degrees C) and pressures (up to 32 MPa). The excess adsorption data are fitted by the supercritical Dubinin-Radushkevich (SDR) model, and the dominant influencing factors of methane adsorption in deep marine shale are determined. Four shale lithofacies are recognized in the study area, including silica-rich argillaceous shale (CM-1), siliceous shale (S), clay-rich siliceous shale (S-3), and argillaceous-siliceous mixed shale (M-2). Nanoscale organic matter (OM) pores, often in irregular, angular and flat shapes, are the dominant pore types in the WF and LMX shale samples. Generally, the pore size spectrum of OM pores is shale lithofacies dependent, e.g., 10-160 nm for S shale and 10-120 nm for CM-1 shale. Compared to other shale lithofacies, S shale exhibits the highest methane adsorption capacity, followed by M-2 shale, while CM-1 shale has the smallest methane adsorption capacity. The adsorption capacity of methane for deep WF and LMX shales is positively correlated with the total organic carbon (TOC) content, micropore volume, and micropore specific surface area. Absolute methane adsorption capacity of deep shale increases with the increase of pressure, but it will decrease at a higher temperature due to the negative and predominant effect of temperature on methane adsorption. The higher TOC content and more abundant small-sized pores promote the S shale to have the strongest adsorption capacity for methane molecules; this indicates that S shale is the most beneficial shale lithofacies for gas adsorption.